The present invention relates to channel access methods for wireless communication systems.
A popular channel access method for wireless Local Area Network (LAN) devices is Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). This method works by having the time since the last activity on the shared channel divided into equal length slots. Each transmitting node in the network generates a random number, then counts the activity slots until the number is reached. At that point, the node can grab the channel and other nodes must suspend their count until the channel is free again. If by chance the two nodes generate the same random number, and thus collide with each other on the channel, they would determine that the packet transfer is unsuccessful through an acknowledgment process and generate a new random number to start counting slots from zero the next time the channel is free. Typically, the range of the random number is increased after a collision. The CSMA system is illustrated with respect to FIGS. 1A, 1B, and 2.
Looking at FIG. 1A, after transmission 22 is finished, a period 24 occurs in which no transmissions are done. After a period 24, the slotted contention window 25 occurs. The transmitting units begin transmitting at a randomly determined contention window slot.
FIG. 1B shows the use of a request-to-send (RTS) and clear-to-send (CTS) signals that help avoid the hidden node problem. A node sends a RTS signal 26 before transmitting. The destination node then sends a CTS signal 28 indicating the length of the data block 30. Nodes out of the range of the source can still avoid transmitting during the time indicated by the CTS signal.
FIG. 2 shows the operation of the contention window. Nodes 1, 2, and 3 all wish to transmit. The nodes randomly generate slot numbers. Node 1 has slot number #5, and it starts transmitting in slot number #5. Nodes 2 and 3 detect the transmissions of Node 1 and thus do not begin transmitting at their randomly selected slot numbers.
The goal of CSMA/CA in most system protocols is to provide fairness of access to the channel for all nodes in the network. This suits computer data networking applications very well. An example where CSMA/CA does not work well is for cordless telephone networks. In this situation, symmetric two-way streams of relatively low bandwidth must be transmitted and received on a regular intervals with very low latency or delays. CSMA/CA has unpredictable and, in theory, unbounded latency.
A common choice for telephone networks is Time Division Multiple Access, or TDMA. In TDMA, the channel is divided into predetermined time slots that are specifically allocated to certain transmitting nodes exclusively on a repetitive basis. The allocation typically requires a node to act as a central controller. TDMA is a cumbersome and inefficient choice for data networking. Most computer network nodes do not need to use the channel at any given time.
Some CSMA/CA and TDMA combination networks have been suggested. None of the suggested methods are effective for the new types of streaming media. Streaming media data includes digital audio player data to one or more digital speakers or headsets. A streaming video stream, such as the picture phone, is another example. It is desired to have a channel access method with low packet failure rate (PFR) and low latency to deal with a wireless LAN environment when both computer data type transmission and streaming data transmissions are used.